Separation Method Using A Column With A Corrugated Cross Structure Packing For Separating A Gaseous Mixture

ABSTRACT

A separation method using a column with cross-corrugated structured packing for separating a gas mixture is presented.

The present invention relates to a separation method using a column withcross-corrugated structured packing for separating a mixture of gases.

Old installations for separating mixtures of carbon monoxide andhydrogen comprise only columns with plates, whereas the new generationof installations of this type uses the technology of columns withcross-corrugated structured packing without a modified interface (EP-A-0837 031). The use of packing in these installations remains tricky, inview of the fact that physical properties that have an impact on theseparation efficiency, the wettability and the foaming behavior, etc.are not comparable to those for air gases. These structured packingshave a local pressure drop at the interface which may be the source of apossible foaming of the mixture to be separated. The presence of foamingimpairs the correct operation of the separation of the various productsto be produced.

The advantage of this invention is in preventing the formation of foamin the portion dedicated to the separation of the mixture. One of theparameters that makes it possible to control the possible formation offoaming may be summarized by the following dimensionless number: S=τm/σwhere τ is the shear stress at the liquid/vapor interface (kgm⁻¹s⁻²), mis the thickness of the film of liquid flowing over the packing (m) andσ is the surface tension at the liquid/vapor interface (kgs⁻²). Thisparameter therefore relates the shear stresses created by the gas overthe liquid with the surface tension of the liquid as described in patentU.S. Pat. No. 5,644,932. It appears that for the applications of thepresent invention, the range of values of this factor S must be between50×10⁻⁶ and 7000×10⁻⁶, preferably between 150×10⁻⁶ and 1500×10⁻⁶.

This parameter can be adjusted in several ways:

-   -   by playing on the geometrical parameters of the packing (within        one and the same section or between two different sections) that        is to say:        -   the angle of inclination δ of the channels relative to the            horizontal (preferably between 30° and) 70°;        -   the crimp angle γ of the corrugations (preferably between            40° and 150°;        -   the density of the packing at least in its central region            (preferably between 300 m²/m³ and 1000 m²/m³);        -   the degree of perforation of the packing (preferably between            3% and 20%);        -   the diameter of the perforations of the packing (preferably            between 1 mm and 4 mm); and        -   the radius of curvature of the corrugation on going from a            central region with corrugations inclined with a given angle            relative to the vertical to a side region where the            corrugations are inclined with a shallower angle relative to            the vertical, or even become vertical (preferably between            0.5 mm and 3 mm);    -   by gradually modifying the angle of inclination of the channels        so as to have channels that are more and more vertical at the        high and low extremities of this angle as described in patent WO        97/16247.

The main advantage of working in these operating ranges that operate byadjusting one or more parameters mentioned previously is an optimizationof the separation capacity or more particularly a reduction in thecolumn diameter for a given separation capacity. It is thereforepossible to considerably reduce the investment costs of the columns andthus of the cold box via these adjustments. The overall reduction in thecryogenic equipment also allows an increase in the flexibility of theunit, which is a second advantage, during the startup and changeoverphases.

According to one subject of the invention, a method is provided for thecryogenic separation of a gas having, as main components, at least twocomponents chosen from one of the following groups: i) hydrogen, carbonmonoxide, nitrogen and methane, ii) nitrogen, oxygen, argon and iii)carbon dioxide, hydrogen, nitrogen, oxygen, argon, the method using atleast one distillation column having cross-corrugated structured packingand/or at least one absorption column having cross-corrugated structuredpacking with at least one section for heat and/or mass exchange betweena descending liquid and an ascending gas, characterized in that theparameter S in this section is between 50×10⁻⁶ and 70 000×10⁻⁶, whereS=τm/σ, τ being the shear stress at the liquid/vapor interface(kgm⁻¹s⁻²), m being the thickness of the film of liquid flowing over thepacking (m) and σ being the surface tension at the liquid/vaporinterface (kgs⁻²).

According to other features of the method:

-   -   at least one packing body in the section for heat and/or mass        exchange comprises a central region and a lower region and        optionally an upper region, the lower region and optionally the        upper region being modified relative to the central region so        that the resistance to the rise of liquid is reduced therein        relative to that in the central region;    -   the gas has, as main components, hydrogen, carbon monoxide,        methane and optionally nitrogen;    -   the gas has, as main components, hydrogen, carbon monoxide and        optionally methane and also nitrogen;    -   the packing is operated with a gas feed F between 0.2-2.5        (Nm⁻²)^(0.5) where F=(ρ_(v)v_(gas))^(0.5) and/or the liquid flow        rate in the packing in L/dm²/h may vary from 50 up to 600        L/dm²/h;    -   the gas that has, as main components, at least two components        chosen from the group: hydrogen, carbon monoxide, nitrogen and        methane in which the parameter S in this section is between        50×10⁻⁶ and 7000×10⁻⁶, more particularly between 150×10⁻⁶ and        1500×10⁻⁶;    -   the gas that has, as main components, at least two components        chosen from the group: nitrogen, oxygen, argon as claimed in one        of the preceding claims 1 to 5, in which the parameter S in this        section is between 500×10⁻⁶ and 70 000×10⁻⁶;    -   at least one packing body in the section for heat and/or mass        exchange comprises a central region and a lower region, the        lower region being modified relative to the central region so        that the resistance to the rise of liquid is reduced therein        relative to that in the central region;    -   the internal diameter of the column varies from one section to        another as a function of the gas and liquid feed for the gas to        be separated;    -   the packing has, at least in one central region, a density of        300 m²/m³ to 1000 m²/m³;    -   at least one characteristic of the packing varies within one and        the same section, the characteristic(s) being chosen from the        group:        -   packing density;        -   angle of inclination;        -   crimp angle;        -   radius of curvature; and        -   degree of perforation,    -   at least one characteristic of the packing varies from one        section to the other, the characteristic(s) being chosen from        the group:        -   packing density;        -   angle of inclination;        -   crimp angle;        -   radius of curvature; and        -   degree of perforation.

The invention will be described in greater detail and referring to thefigures, in which

FIG. 1 illustrates a methane scrubbing process according to theinvention and

FIG. 2 illustrates a partial condensation process according to theinvention,

FIG. 3 schematically represents a view of the corrugations in the axisof the waves,

FIG. 4 represents a schematic top view of the corrugated lamella,

FIG. 5 represents a method for separating carbon dioxide according tothe invention and

FIG. 6 represents a method for separating air according to theinvention.

In the methane scrubbing systems (FIG. 1), the syngas which is underpressure and cooled to −180° C., is scrubbed with liquid methane in thecolumn K01 operating at high pressure (between 12 and 50 bara) and atemperature as low as possible, carbon monoxide is entrained in thebottom of the column and hydrogen is produced at the top.

The column K01 contains at least one packing body as described in WO97/16247. The use of such a packing having a modified interface isparticularly advantageous since hydrogen, having a very low pressuredrop compared to other gases, allows an operation at very high gas feedwithout significant degradation in terms of separation efficiency. Theless abrupt change at the interface between sections with packing havinga modified interface makes it possible to operate with a more constantparameter S, which reduces the risk of foaming at the interface betweentwo packing bodies and makes the operation of the column in the steadystate and in the changeovers more reliable.

The dissolved hydrogen is then discharged into the medium-pressure flashcolumn K02. The CO/CH₄ binary mixture is then separated in thelow-pressure distillation column K03. Gaseous CO is produced at the top,the liquid methane produced at the bottom being pumped and recycled forthe scrubbing operation in K01.

The refrigerating capacity is produced in a CO cycle.

The other columns K02, K03 may also contain cross-corrugated structuredpacking with modified or unmodified interface(s).

All the columns operate with a factor S between 50×10⁻⁶ and 3000×10⁻⁶,more particularly between 150×10⁻⁶ and 1500×10⁻⁶.

In the systems with partial condensation (FIG. 2), the syngas, scrubbedof its methane in K11 by the liquid CO originating from B01, is cooledto the lowest possible temperature, the limitation being thesolidification temperature of the CO; the liquid condensed in B02 ispredominantly CO.

The flash columns K12 and K13 remove the dissolved hydrogen respectivelyin the bottoms liquids of K11 (rich in methane) and the liquid from thepot B02 (rich in CO).

The liquids from columns K12 and K13 then feed the distillation columnK14 where the CO/CH₄ separation is carried out. In the CO/CH₄ separationcolumn (K14), which has at least two packing sections operating at verydifferent refluxes, the use of packing sections having a differentdensity and/or angle of inclination makes it possible to optimize theparameter S for the whole of the column.

The other columns K02, K03 may also contain cross-corrugated structuredpacking having modified or unmodified interface(s).

The refrigerating capacity is obtained by hydrogen expansion inturbines.

In the particular case where nitrogen is present, an N₂/CO separationcolumn could be added downstream of the CO/CH₄ column.

All the columns operate with a factor S between 50×10⁻⁶ and 7000×10⁻⁶,more particularly between 150×10⁻⁶ and 1500×10⁻⁶.

Given below is an example of the calculation of the factor S for amethane scrubbing column.

Linear pressure drop for impure hydrogen with 1.2 mbar/m in a structuredpacking having a density of 650 m²/m³:

$\tau = {{\frac{D_{h}}{4} \cdot \frac{\Delta \; P}{\Delta \; L}} = {{{\frac{0.0062\mspace{20mu} m}{4} \cdot 120}\mspace{20mu} \frac{N}{m^{3}}} = {0.186\mspace{20mu} \frac{N}{m^{2}}}}}$

The thickness of the film for laminar flow of liquid methane at 93 K inthe ascending hydrogen is:

$\begin{matrix}{m = \left\lbrack \frac{3 \cdot \mu_{L} \cdot \Gamma}{\rho_{L} \cdot \left( {\rho_{L} - \rho_{V}} \right) \cdot g} \right\rbrack^{1/3}} \\{= \left\lbrack \frac{{3 \cdot 0.0021}\mspace{14mu} {\frac{kg}{m \cdot s} \cdot 0.0052}\mspace{14mu} \frac{kg}{m \cdot s}}{460\mspace{14mu} {\frac{kg}{m^{3}} \cdot \left( {{460\mspace{14mu} \frac{kg}{m^{3}}} - {7.7\mspace{14mu} \frac{kg}{m^{3}}}} \right) \cdot 9.81}\mspace{14mu} \frac{m}{s^{2}}} \right\rbrack^{1/3}} \\{= {0.000117\mspace{14mu} m}}\end{matrix}$

For liquid methane at 93 K, the surface tension σ=0.018 N/m

$S = {\frac{\tau \cdot m}{\sigma} = {\frac{0.186\mspace{14mu} {\frac{N}{m^{2}} \cdot 0.000117}\mspace{14mu} m}{0.018\mspace{14mu} \frac{N}{m}} = {1209*10^{- 6}}}}$

Verification of laminar flow conditions in a falling film of liquid withRe_(L)<2000

${Re}_{L} = {\frac{4 \cdot \Gamma}{\mu_{L}} = {\frac{{4 \cdot 0.0052}\mspace{14mu} \frac{kg}{m \cdot s}}{0.00021\mspace{14mu} \frac{kg}{m{\cdot s}}} = 99}}$

Legend

τ shear stress at the vapor/liquid interface (N/m²)

σ surface tension of the liquid (N/m)

γ liquid mass flow per unit of width of the exchange surface area(kg/m/s)

μ_(L) dynamic viscosity of the gas (kg/m/s)

ρ_(L) density of the liquid (kg/m³)

ρ_(v) density of the vapor (kg/m³)

D_(h) hydraulic diameter of the structured packing channel (m)

ΔP/ΔL linear pressure drop of the gas in the vertical direction (N/m³)

m film thickness of the liquid (m)

g gravitational constant (9.81 m/s²)

S dimensionless parameter that characterizes the internal and externalforces at the vapor/liquid interface

Re_(L) Reynolds number of the falling film of liquid (dimensionless)

FIG. 3 shows a packing lamella having a corrugation of height H with acrimp angle γ.

Represented in FIG. 4 is a lamella 1, having oblique parallelcorrugations, of which the crests 2 are represented as thick lines andthe troughs 3 as thin lines.

The inclination of the corrugations is defined by the angle δ formedbetween the wave crest 2 and the lower edge 4 in the central region C.An upper region S going from the upper edge 4 a of the element to theupper limit of the central region C and in a lower region I going fromthe lower edge of the element to the lower limit of the central regionC, each region S, I having a height h′. The angle formed between thecrests of the waves and the edge 4 is δ₁=90° but may have other values.

Seen in FIG. 5 is the process for separating carbon dioxide bydistillation in accordance with the invention. A stream 1 of carbondioxide mixed with nitrogen, oxygen and argon is cooled in an exchanger3, separated in a distillation column 5 having an overhead condenser anda bottoms reboiler. The bottoms liquid 7 from the column 5 is sent to acolumn 9 having an overhead condenser and a bottoms reboiler. The column9 produces at the top a stream 11 and at the bottom a stream 13. Thestream 13 is the liquid product that is rich in carbon dioxide and thestream 11 contains nitrogen, argon and oxygen. This process is describedin detail in EP-A-503910. Other examples of units that can be operatedaccording to the method of the invention are given in patentapplications U.S. 60/890233, U.S. Ser. Nos. 11/695,422, 11/695,446,11/695,455 and 11/695,471.

FIG. 6 illustrates a double air separation column that operatesaccording to the method of the invention. Other types of columns mayalso function according to the invention, such as single columns, mixingcolumns, triple columns, argon separation columns, etc.

The cooled, compressed and purified air 601 is sent to the bottom of amedium-pressure column 605 thermally coupled to a low-pressure column609. Reflux streams 607, 603 are sent from the medium-pressure column tothe low-pressure column. Streams rich in oxygen 613 and rich in nitrogen611 are withdrawn from the low-pressure column.

1-6. (canceled)
 7. A method for the cryogenic separation of a gashaving, at least two components selected from one of the groupsconsisting of: i) hydrogen, carbon monoxide, nitrogen and methane; ii)ii) nitrogen, oxygen, argon; and iii) iii) carbon dioxide, hydrogen,nitrogen, oxygen, argon; the method using at least one distillationcolumn having cross-corrugated structured packing, at least oneabsorption column having cross-corrugated structured packing with atleast one section for heat and/or mass exchange between a descendingliquid and an ascending gas, or a combination thereof, wherein theparameter S in this section is between 50×10⁻⁶ and 70 000×10⁻⁶, whereS=τm/σ, τ being the shear stress at the liquid/vapor interface(kgm⁻¹s⁻²), ρm being the thickness of the film of liquid flowing overthe packing (m), and σ being the surface tension at the liquid/vaporinterface (kgs⁻²).
 8. The method of claim 7, further comprising at leastone packing body in the section for heat and/or mass exchange, andwherein said at least one packing body comprises a central region and alower region and an upper region, the lower region and the upper regionbeing modified relative to the central region so that the resistance tothe rise of liquid is reduced therein relative to that in the centralregion.
 9. The method of claim 7, wherein the gas comprises at leasthydrogen and carbon monoxide.
 10. The method of claim 9, wherein the gasfurther comprises methane and nitrogen.
 11. The method of claim 7,wherein the packing is operated with a gas feed F between 0.2-2.5(Nm⁻²)^(0.5) where F=(ρ_(v)v_(gas))^(0.5) and/or the liquid flow rate inthe packing in L/dm²/h may vary from 50 up to 600 L/dm²/h.
 12. Themethod of claim 7, wherein the gas that has at least two componentsselected from the group consisting of hydrogen, carbon monoxide,nitrogen and methane, wherein the parameter S in this section is between50×10⁻⁶ and 7000×10⁻⁶,
 13. The method of claim 12, wherein the parameterS in this section is between 150×10⁻⁶ and 1500×10⁻⁶.
 14. The method ofclaim 7, wherein the gas that has at least two components selected fromthe group consisting of nitrogen, oxygen, argon, wherein the parameter Sin this section is between 500×10⁻⁶ and 70 000×10⁻⁶.